Legionellosis (Legionnaires' Disease)

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Disease Condition


Legionella species are Gram-negative bacilli commonly found in water. There are over 50 species and ~70 serogroups currently recognized. L. pneumophila serogroup 1 is primarily responsible for human disease.


Transmission occurs by breathing in small droplets of water in the air that are contaminated with the Legionella bacteria. An example is breathing in steam from a contaminated hot tub. Transmission may also occur by aspirating contaminated water. No human-to-human transmission occurs.


There are two distinct clinical manifestations of disease:

  • Legionnaires’ Disease presents as pneumonia with a cough. Other symptoms may include a high fever, chills, myalgia, muscle aches, and/or headache. Abdominal pain, nausea, vomiting, and diarrhea are also common, and some people may have confusion or other mental changes.  
  • Pontiac Fever presents as a self-limited febrile illness that does not result in pneumonia. Symptoms may include fever, headache, malaise, and muscle aches.

The key to preventing legionellosis is maintenance of the water systems in which Legionella grow, including drinking water systems, hot tubs, decorative fountains, misters, and cooling towers. Cooling towers should be drained when not in use and mechanically cleaned periodically to remove scale and sediment. Appropriate biocides should be used to limit the growth of Legionella and the formation of protective biofilms. Maintaining hot water system temperature at 122°F or higher may reduce the risk of transmission. Tap water should not be used in respiratory therapy devices.

There are no vaccines that can prevent legionellosis. Persons at increased risk of infection may choose to avoid high-risk exposures, such as being in or near a hot tub.

Recent Texas Trends
From 2010 to 2019, the number of legionellosis cases reported to DSHS gradually increased from a low of 111 cases in 2011 to a high of 421 cases in 2019, followed by a drop to 316 cases in 2020.  

In Texas, the highest age-specific rates of legionellosis are typically found among middle aged and older adults. Although legionellosis cases are rarely reported in children in Texas, two cases of legionellosis were reported in 2019.